Optical semiconductors (LEDs) have rapidly become widespread in recent years because of having advantages of lower power consumption and longer life, in comparison with conventional light-emitting devices such as light bulbs and fluorescent lamps. When optical semiconductor devices are manufactured, a curable resin composition, called a die attach material, which is used for the purpose of fixing an optical semiconductor chip on a substrate is first applied at a predetermined position on the substrate of the optical semiconductor. As methods for applying a die attach material, two methods are commonly used, a dispensing method in which a resin filled in a syringe is discharged from a needle attached to the tip of the syringe, by applying pressure, to transfer it onto a substrate, and a stamping method in which a transfer pin is pressed into a resin which has been spread in a thin film on a resin plate, and then the pin with the resin attached thereon is pressed onto a substrate, thereby to transfer the resin onto it. Subsequently, a die bonding process is performed in which on the top of the applied resin composition is pressure-bonded an LED chip which has a most portion of its bottom part formed of sapphire, followed by a process in which the resin composition is cured. After that, a wire bonding process is performed in which the electrode pad portions, mostly formed of gold, of the optical semiconductor chip and the electrically conductive lead frame portion, mostly formed of silver, on the substrate are bonded by a gold wire. In the wire bonding process, the gold ball formed at the tip portion of the capillary is pressed against the electrode pad portion while applying ultrasonic waves. In this process, when the LED chip is not sufficiently fixed on the substrate, there is caused a problem that ultrasonic wave diffuses to the surroundings during the pressing, whereby the gold wire cannot be bonded with sufficient strength. Further, when after the LED device has been manufactured through post-processes, the LED chip floats from the substrate during the lighting operation, heat dissipation from the LED chip, which is a heating element, to the substrate cannot be sufficiently achieved, giving rise to a possibility that malfunctioning of the LED device is caused. For these reasons, a curable resin composition capable of giving a cured product with high strength and high adhesiveness is employed for a die attach material, in order that an LED chip is sufficiently fixed on a substrate.
Additionally, addition-curable silicone resin compositions such as ones derived from methylsilicones are widely used as a die attach material for high-power blue LED devices that frequently are used in lighting applications, from the viewpoint of imparting heat resistance and light resistance. However, it has been reported that contaminants are formed on gold electrode pad portions of an optical semiconductor chip during the curing of the resin composition, due to various factors such as the type of the addition-curable silicone resin composition and optical semiconductor chip used and the conditions under which the resin composition has been cured. Contaminants that are present on electrode pad portions give rise to problems because of causing adverse effects in the subsequent wire bonding process. Such contaminants are ascribed to low molecular weight siloxanes contained in the addition-curable silicone resin composition, and it is a problem that in particular, when the resin composition is subjected to curing, low molecular weight siloxanes having a hydrogen atom bonded to a silicon atom (SiH group) are heated, leading to volatilization and diffusion, so that they undergo hydrolysis reaction or the like, thereby resulting in the formation of a film, or the generation of gel, which adheres as contaminants, on the electrode pad portion. It is known that decreasing the content of low molecular weight siloxanes having a SiH group in the resin composition makes it possible to reduce the amount of contaminants to the LED chip electrode pad portion that are attached during the heat curing of the composition, leading to an improvement in the wire bonding property.
A die attach material is usually used such that the transferring of it onto a substrate and crimping of an LED chip are performed in an amount at which a fillet portion is formed around the chip, in order to reinforce the fixation of the LED chip to the substrate. When the resin has a low level of retention on the substrate, the fillet portion thinly spreads more widely to the surroundings immediately after the chip is crimped. In such a case, since the surface area of the resin portion becomes larger, a larger amount of SiH group-containing low molecular weight siloxanes in the resin composition is prone to volatilize during the curing, resulting in an increased level of adhesion of contaminants to the electrode pad portion of the LED chip.
Furthermore, addition-curable silicone resin compositions such as ones derived from methylsilicones are inferior in the adhesiveness to substrates, relative to conventional epoxy resins, due to the polarity of the resin and others. In particular, in comparison of the sapphire at the bottom of an LED chip, which is an adherend to a die attach material, and the silver lead frame portion, there is an increased tendency that the adhesiveness of the resin to the latter is poor. Usually in an LED package, a white resin called a reflector is molded in a lead frame. In cases when for a reflector member, use is made of an epoxy molding compound (hereinafter referred to as an “EMC”) which has an even higher adhesiveness to a molding die in comparison with thermoplastic resins such as polyphthalamides, a predetermined amount of a wax is commonly added in order to improve mold release characteristics. The LED package molded with an EMC undergoes a chemical etching process with an acid or a base, or a physical etching process by blast cleaning, etc. However, when the wax component that has oozed out to the lead frame during the molding process is not sufficiently removed by the etching process, the adhesiveness between the lead frame portion and the die attach material becomes deteriorated.
In JP-A 2013-254893 (Patent Document 1), it has been reported that by using an LED chip in which a metal protective layer containing Ni or Ta is provided on the gold electrode portion, good wire bonding properties can be achieved also in cases of using a die attach material which is made of an addition-curable silicone resin causing a high level of electrode contamination. However, LED chips having such a protective layer have a higher manufacturing cost than common LED chips having no protective layer, so that there is a need for a die attach material itself that results in a reduced level of electrode contamination.
In JP-A 2016-122828 (Patent Document 2), it has been reported that the light-emitting efficiency of an LED device is improved by using a sulfur-derived brightener on the silver lead frame of an LED package molded with an EMC. However, when such a brightener is used on the silver surface of the silver lead frame, there is a possibility that the adhesiveness of the die attach material is reduced, and thus it is required that a die attach material be provided with an even higher adhesiveness to a substrate.
For example, it has been reported in JP-A 2008-255227 (Patent Document 3) that in an addition-curable silicone resin composition, the low molecular weight siloxane component contained in the resin composition includes many low molecular weight siloxanes containing a SiH group, which is a reactive group, in addition to nonfunctional low molecular weight siloxane components represented by D3 and D4. It has been described that especially by setting the content of low molecular weight siloxane compounds having a degree of polymerization of 10 or less and containing one or more SiH groups per molecule to be a given weight % or less of the entire silicone resin composition, the adhesion of contaminants to the surroundings during the heat curing of the composition can be prevented. However, the addition-curable silicone resin compositions described in the examples have concerns about adhesion strength when used as a die attach material for LED devices, because the cured materials derived from these compositions are soft, wherein their hardness as defined in JIS K 6249 and measured with a type A durometer is 32 to 66.
In JP-A 2012-12434 (Patent Document 4), it has been reported that an addition-curable silicone resin composition that is a combination of a linear organopolysiloxane, wherein the a type D durometer hardness as defined in JIS K 6253 is 30 to 70 and the organohydrogensiloxane component has a SiH group in the side chain, and a branched organopolysiloxane which has a SiH group at a terminus thereof is superior in light transmission and durability of adhesiveness. However, when the composition is used as a die attach material for an LED device, JP-A 2012-12434 is silent on the contamination to the gold pad portion on an LED chip during the curing of the composition.
As described above, conventional addition-curable silicone resin compositions are problematic: when they are used especially as a die attach material for an LED device, there are caused problems that contaminants adhere to the electrode pad portion on an LED chip during the curing of the composition, resulting in a decrease in the wire bonding property, and they bring about an insufficient adhesiveness to the silver lead frame portion on an optical semiconductor substrate molded with a reflector member.